A typical image sensor includes various optical and electronic elements formed on a front side of the sensor. The optical elements include at least an array of individual pixels to capture light incident on the image sensor, while the electronic elements include transistors. Although the optical and electronic elements are formed on the front side, an image sensor can be operated as a frontside-illuminated (FSI) image sensor or a backside-illuminated (BSI) image sensor. In an FSI image sensor, light to be captured by the pixels in the pixel array is incident on the front side of the sensor, while in a BSI image sensor the light to be captured is incident on the back side of the sensor.
Compared to FSI image sensors, BSI image sensors drastically improve fill factor, quantum efficiency and cross talk, hence improving the sensor's overall optical performance. BSI technology also makes it possible to continuously scale CMOS pixel size down to sub-0.11 microns. However, unlike FSI, BSI blooming issues have not been satisfactorily solved due to three major obstacles. First, BSI sensors intrinsically have no highly-doped bulk region to recombine extra photoelectrons. Next, BSI outperforms FSI for pixel sizes of 1.75 micron and below, but unlike FSI there is no room to add anti-blooming features into the already very small pixel cells. Finally, BSI image sensors collect photons from the back side, but the silicon substrate in a BSI sensor is only about 2 microns thick, there is no room vertically to design vertical overflow drains between the back side and the photodetector to capture the extra photoelectrons.